docs: def on_unsubscribe(self):
          # type: () -> Callable
          """Decorate a callback funtion to handle unsubscribtions.
  
          **Usage:**::
  
              @mqtt.unsubscribe()
              def handle_unsubscribe(client, userdata, mid)
                  print('Unsubscribed from topic (id: {})'
                        .format(mid)')
          """
          def decorator(handler):
              # type: (Callable) -> Callable
              self.client.on_unsubscribe = handler
              return handler
  
          return decorator